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Howesight
Samba Member
Joined: July 02, 2008
Posts: 3401
Location: Vancouver, B.C.
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 Posted: Tue Sep 05, 2023 7:27 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Hi Pchill and Andy Bees:
I think that the cocking of the 4th gear occurs not due to tapering wear (hence little or no evidence of gear bore wear), but due to the designed-in clearance, plus, when gearboxes accrue miles, the additional clearance caused by wear on all surfaces and on the needle rollers too.
Since the ID of the mainshaft bearing and the thrust surface of 4th gear turn at the same nominal rotational speed, there really is no other available explanation. (I say nominal because there may be some harmonics in the gear mesh and/or arising from the engine.) The open question is really how much of that erosion is due to cocking and how much is due to pure axial thrust forces. I don't think it matters since straight-cut gears will eliminate both forces. With both axial and cocking forces eliminated, oil can occupy the space between the mainshaft bearing ID and 4th gear, instead of being squeezed out. Then, (assuming my theory is correct), the metal erosion should not occur.
My thought is that the brinelling and false brinnelling are likely not a serious concern for most of us because most of the transaxle's life occurs in 4th gear, where the condition of the gear bore and needle rollers are not so critical. If the brinelling were to be found on 3rd gear, for example, which is spinning on the mainshaft most of its life, brinelling of either flavour would be rather critical.
Rather, the brinnelling and false brinelling are mainly some evidence of the beyond-design-parameter forces involved when re-powering. Even with straight-cut gears of the same material, we might very well see these brinelling clues. But the GT gears are made of better alloy and hopefully will not experience any brinelling.
To Andy, I don`t think that the engagement of 4th gear through the slider and thence to the drive hub and thence to the mainshaft is sufficient to lock 4th gear in a way that prevents 4th gear from cocking nor from the oscillating I have been struggling to describe adequately. In a nutshell, I think that the 4th gear, when high torque is applied to it when in gear,
1. is not perfectly concentric with the mainshaft due to bearing clearance; and
2. Is not perfectly vertically aligned, but instead, is cocked at a slight angle, due to the axial forces applied on one side of the gear - - specifically the side that is in mesh at any time with the fixed 4th gear on the countershaft. It may help in visualizing this to think of a swash plate at a very small angle of inclination.
Regarding the rumbling of the transaxle at idle speed in neutral, for what it is worth, my transaxle has done this from day one after the 2012 rebuild, once the gearbox is up to operating temperature. On cold starts, the Swepco 210, like a fine molasses, quiets all the gearbox noises.
If anyone has installed the straight-cut gears and had a chance to examine the interface between 4th gear and the mainshaft bearing after putting some miles on the gearbox, please chime in.
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'86 Syncro Westy SVX |
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Pchill2
Samba Member
Joined: November 19, 2021
Posts: 410
Location: Georgia
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| Posted: Tue Sep 05, 2023 8:55 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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I offered up an explanation for the wear shown in the pictures in a previous post. I believe the wear shown could have nothing to do with driving.
What people forget is that it takes the same amount of power to make the van go 80 mph regardless of how much you have. Getting to 80 faster is a different story but that’s not what’s discussed in this thread.
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87 Syncro Westy
Honda K24 swap |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Tue Sep 05, 2023 9:31 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Pchill2 wrote: |
| What people forget is that it takes the same amount of power to make the van go 80 mph regardless of how much you have. Getting to 80 faster is a different story but that’s not what’s discussed in this thread. |
Yes 80 with all things equal, is equal.
"Sustaining 80" is different. There are grades, headwinds.
You can get to 80 with a WBX, but it has a tailwind or a downhill. That is not the same as 80mph in a chipped diesel that encounters an uphill and remains at 80mph.
If you had
a WBX which can 'go 80',
-and a chipped TDi which can "sustain 80",
-and torque meter,
-and a way to measure the accumulated torque>vs>time
.....there would be a vast difference.
The "area under the torque>vs>time curve" reflects the wear on the gearbox.
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb |
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Pchill2
Samba Member
Joined: November 19, 2021
Posts: 410
Location: Georgia
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| Posted: Tue Sep 05, 2023 10:02 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Howesight wrote: |
Regarding the rumbling of the transaxle at idle speed in neutral, for what it is worth, my transaxle has done this from day one after the 2012 rebuild, once the gearbox is up to operating temperature. On cold starts, the Swepco 210, like a fine molasses, quiets all the gearbox noises.
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An 80W140 is way to heavy for these gearboxes. Improper viscosity oils are a leading contributor to both false brinelling and corrosion fretting.
https://www.stle.org/images/pdf/STLE_ORG/BOK/LS/Be..._Dec03.pdf
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87 Syncro Westy
Honda K24 swap |
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Pchill2
Samba Member
Joined: November 19, 2021
Posts: 410
Location: Georgia
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| Posted: Tue Sep 05, 2023 10:29 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Sodo wrote: |
| Pchill2 wrote: |
| What people forget is that it takes the same amount of power to make the van go 80 mph regardless of how much you have. Getting to 80 faster is a different story but that’s not what’s discussed in this thread. |
Yes 80 with all things equal, is equal.
"Sustaining 80" is different. There are grades, headwinds.
You can get to 80 with a WBX, but it has a tailwind or a downhill. That is not the same as 80mph in a chipped diesel that encounters an uphill and remains at 80mph.
If you had
a WBX which can 'go 80',
-and a chipped TDi which can "sustain 80",
-and torque meter,
-and a way to measure the accumulated torque>vs>time
.....there would be a vast difference.
The "area under the torque>vs>time curve" reflects the wear on the gearbox. |
For sure.
I do agree that the corrosion fretting of the gear thrust surface is caused by lack of lube and relative motion.
I am posing an alternate cause of this failure. The engine at idle probably vibrates around 25Hz/0.4g. Easy to see how a loose input shaft bearing would cause the input shaft to rattle like hell and couple that with a giant hollow tube running to the front diff acting as an amplifier could cause some damage.
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87 Syncro Westy
Honda K24 swap |
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AndyBees
Samba Member
Joined: January 31, 2008
Posts: 2621
Location: Southeast Kentucky
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| Posted: Wed Sep 06, 2023 6:44 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Well, guys, back in 2010-12, when I built my Transmission and putting my TDI project together, all this stuff was unknown to me. Other than rebuilding three 010 VW Automatic Transmissions, I never had any experience and not been inside a manual transmission of any kind.
So, considering my build has well over 88k miles on it with more than 25k of those pulling a 1200 lb camper and loads of them on cruise at 70 mph, it will certainly be interesting to see what it looks like inside.
_________________
'84 Vanagon Tin-top, ALH TDI, two trips to Alaska, 2014 & 16. 1989 Tin-top unmolested.
1983 Air-cool, 225k miles, 180k miles mine, seven trips to Alaska from 1986 thru 2003. 1975 Bay hopeful. |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Wed Sep 06, 2023 12:03 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Sorry been busy ( been enjoying a Syncro Traveller driveway camping, and flogging his machine to maintain roadworthiness).
Can't answer these technical questions on the phone....gotta get the laptop out.
I will work thru the discussion.
| Pchill2 wrote: |
| Good post Howesight but I'm not on board with "cocking" of the gear. The axial force relatively uniform across the mesh. It would be visibly evident if the contact mesh was skewed towards one side of the gear and the bore would be tapered. |
Yep, have seen lots of brinelling, thrust erosion, no evidence of gear cocking. If there was gear cocking, the erosion lines would be wider on the ends, narrow in the middle, but they are straight, and equal width beginning to end.
The eroded thrust faces are 'square'.
| Pchill2 wrote: |
I do agree with the loose fit of the gear and bearing to the shaft. The false Brinelling on the inside of the gear bore shows a stationary vibration. The 4th gear is always spinning when the vehicle is in motion.
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When "in 4th" the gear is spinning, but it's locked to the shaft thus the needle is stationary in the bore, and on the shaft. But the bearing itsef precesses due to diametral slop. So the needle bearing does rotate, but very slowly. Someday I should measure the slop and figure out the rotational speed (for fun) but for now, logically the bearing should rotate by precession while in 4th. Which is GOOD.
......Until the brinnelling gets too deep, then precession cannot climb the needle out of the "line". Then next time, --->if<--- the needle stops in "the line" it stays there and continues to erode the line deeper ----> after the oil depletes.
I don't see a situation where these can get brinnelled while stationary. Think of a "dry gearbox" with a tag stating "no oil" and transported by railcar across the continent. There would be assembly lube which should prevent that. And there's no centrifugal force to deplete the lube. I don't see it happening due to transport. Not on our gearboxes. And if it was only an 'image' in the sheen, it would disappear, burnish away in-use.
| Pchill2 wrote: |
| Couple this with a ton of folks reporting noises when idling in neutral and it makes me lean towards some harmonic vibration. |
I suspected some kind of unexpected violent condition (like resonance) for years. But we could never find any indication. Then corrosion fretting came along and it has too many telltale signs to let it go. It's our current darling.
Corrosion fretting does not occur in the presence of lubrication. It must be "metal-to-metal", =dry. Idling 800 RPM, in neutral, is super-lubrication.
Noise in neutral is generally the mainshaft bearing. It can make noise once it's wallowed out its bore. Then it can't transfer heat, and the bearing just wears faster. And there's the electrical assault happening in this 3rd decade of corroded steel bolts in the magnesium cases. And in a Syncro neutral noise can be the granny idler gears just chattering loose. Gear chatter sounds similar but different than a bearing.
| Pchill2 wrote: |
| If the false Brinell marks were actually Brinell damage it would have been due to shock loading. Until they are measured I assume they are from stationary micro movements from vibration. As could be the wear on the thrust face. |
Doubt there is a shock load that could produce a "real brinnelling" that is accurately consistent around the shaft and bore. It would be on one side of the shaft.
| Pchill2 wrote: |
I do agree with Sodo on the lack of lubrication on the thrust face but don't think that is correlated big engines due to the nature of the failure.
Many Vanagons fail with stock engines. |
I think it is related to mashing on the pedal for hours, no letup, ---->and the subsequent oil depletion. The big engine would just thrust harder and thus erode the thrust face quicker.
And as the theory goes, the WBX periodically downshifts, which floods 4th with oil, stopping the erosion (until it depletes again).
While the big engine remains in 4th for longer as the driver wears a smile. 
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb
Last edited by Sodo on Wed Sep 06, 2023 4:33 pm; edited 4 times in total |
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Pchill2
Samba Member
Joined: November 19, 2021
Posts: 410
Location: Georgia
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| Posted: Wed Sep 06, 2023 12:56 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Sodo wrote: |
I don't see a situation where these can get brinnelled while stationary.
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It cannot get brinelled while stationary. It can get false brinelled. These have two different failure modes.
| Sodo wrote: |
Idling is super-lubrication.
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Not while the clutch is depressed. The 4th gear is high and dry as is the mainshaft bearing.
Couple that with folks using some super high viscosity lubricants and you'll likely not be seeing much oil in the gear anyway.
| Sodo wrote: |
Noise in neutral is generally the mainshaft bearing.
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This is what I'm saying is causing the damage. Vibration of the gear face on the mainshaft bearing due to poor fits.
You asked for some independent research. From the threads I've seen the common component is noise near the mainshaft area of the transmission while idling.
Howesight claims to have a noise while idling cold. Perhaps push the 4th gear into the mainshaft bearing with a screwdriver and listen for the noise to change?
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87 Syncro Westy
Honda K24 swap |
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dobryan
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Joined: March 24, 2006
Posts: 17127
Location: Brookeville, MD
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tjet
Samba Member
Joined: June 10, 2014
Posts: 3725
Location: Az
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| Posted: Wed Sep 06, 2023 1:48 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Great post & video.
I'm looking forward to installing and trying out my rebuilt syncro trans with GT slider hub and .925 standard cut 4th. Paul said that the .925 doesn't require a straight cut.
Powered by a slightly higher output 2.2 WBX |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Wed Sep 06, 2023 2:11 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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I am welcoming an independent lookie at all this !
Skimmed but haven’t digested the windmill analysis yet on stle.org. You can bet that I WILL !!
I intend to digest MarkWards, Howesight's posts too. Will be great to see in AndyBees 88k gearbox.
| pchill2 wrote: |
| If you cannot measure the indents on the bore of that bearing then what you have is actually false brinelling, not brinelling. This is a completely different failure mode... |
If you settled the needle into the indents, I bet you could measure the depth of the lines with a dial gauge.
You can definitely "catch a fingernail" on these "longitudinal erosions" that we call "false brinnelling".
I do not believe there can be any 'impact brinnelling' in a VW gearbox. Even chunks passing thru the gear teeth doesn't produce "impact" brinnelling. Thus any brinnelling is likely "false brinnelling".
Laziness causes poor communication. We should always be using the term False Brinnelling in the VW gearbox discussion.
Realistically I don't even know if false brinnelling is correct. False Brinnelling may even refer to a "sheen" rather than an actual depression.
We need a genuine drivetrain expert on board to clear up our terminology.
| Quote: |
| Not while the clutch is depressed. The 4th gear is high and dry as is the mainshaft bearing. |
Loads are near-zero when the clutch is depressed. Bearings can last almost indefinitely at near-zero loads.
I have not heard yet of too-thick oil causing oil starvation damage. I'm curious how 'thin' it gets at a basic temp of 140°F. Cavitation erosion is a concern of too-thick oil.
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb
Last edited by Sodo on Wed Sep 06, 2023 4:53 pm; edited 1 time in total |
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E1
Samba Member
Joined: January 21, 2013
Posts: 8244
Location: Westfalia, Earth
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| Posted: Wed Sep 06, 2023 2:34 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Curious about the “thick oil” comment myself. 75-140 synthetic for me, though we rarely see freezing temps.
And when it is cold, a reasonable amount of mechanical sensitivity makes it clear it doesn’t want to be run and shifted yet.
If the cold box feels stiff, I give it a couple to warm and loosen up and am very Ginger on it until it shifts freely — despite being a far-bigger fan of Mary Ann (with apologies for confusing the youngsters).
_________________
1984 Westfailure/2.1 Digijet/5.43 Ring & Pinion/Peloquin/D-rated BFG KO2s
AI has spoken to further illiteracy, to steal, to cheat, and to replace humans
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere." — Colin Chapman |
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Howesight
Samba Member
Joined: July 02, 2008
Posts: 3401
Location: Vancouver, B.C.
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| Posted: Wed Sep 06, 2023 6:06 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Pchill2 wrote: |
| Howesight claims to have a noise while idling cold. Perhaps push the 4th gear into the mainshaft bearing with a screwdriver and listen for the noise to change? |
Pchill:
We appear to be miscommunicating. My transaxle is very quiet when the gear oil is cold. Only when the gear oil is good and hot will I hear the gearbox noise I referred to and this is with the engine running at idle, transaxle in neutral, and clutch engaged, thus spinning the mainshaft. The sound is not audible in the vehicle, but is clearly audible outside the vehicle.
My hope is to proactively (as preventive maintenance) rebuild this gearbox myself when life throws some spare time at me. If that happens, I will document my findings to help add to the collective experiences we have had. Seeing Herman's, Sodo's and AndyBees' rebuild pics (I see Andy on the TDi website too), Paul Guard's many posts and many others I can't summon to mind at the moment, I am inspired to give this a try. It's less of a mystery now. Just need to find that time and a few tools.
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Wed Sep 06, 2023 8:52 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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| Pchill2 wrote: |
| 4th gear should have lubrication grooves cut into the thrust face. Better yet it would be easy enough to turn the face down to work with a grooved bronze washer. |
Grooves could allow the oil to be thrown out (thus deplete) faster. Rancho Transaxles has experimented with this.
| charleslabri wrote: |
Killer post!
Did the front ground delete apply to 2wd? |
Thx! Yes the delete applies to VW van and Bus gearboxes. Or any gearbox that has a "daisy-chain" of dissimilar metals electrical connections between the starter (or alternator) and chassis. There is no benefit, only detriment to current flow that risks crossing precision rolling elements.
It's more than a "delete", your starter still NEEDS a ground. Replace the fwd ground with a copper cable direct from the starter to the chassis and your starter will run more reliably as well. And direct copper from alternator case to chassis further protects the gearbox, and offers the benefit of "no lost volts".
The ideal configuration is your gearbox and engine ISOLATED from the two high-amperage electrical components (alternator and starter). All the other grounding needs are tiny in comparison and fall in the shadow of the two biggies.
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb
Last edited by Sodo on Thu Sep 07, 2023 6:02 pm; edited 4 times in total |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Wed Sep 06, 2023 10:05 pm Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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The ^^gear cocking^^ theory illustrated from Samba discussions circa 2017.
| Howesight wrote: |
| 2. There are also unequal forces on the 4th gear idler (Paul Guard referred to this as "gear cocking"; |
the problem with the gear cocking theory is that evidence suggests it does not occur. The 'false brinnelling' lines would be narrower in the center than on the ends if there was gear cocking. But they are perfectly parallel.
And the thrust erosion is perpendicular.
| Howesight wrote: |
| 3. Due to the bearing clearance between the mainshaft, rollers and inside diameter (ID) of the 4th gear idler, the idler gear itself can move in an oscillating manner best envisioned by thinking about a hulu hoop in action on somebody's waist; |
I've been calling that motion 'precession'. It's amplitude is equal to the bearing clearance.
| Howesight wrote: |
| 5. If you think of the idler gear as cocked (not running perfectly vertical) while the thrust surface of the mainshaft bearing remains truly vertical (or very close to vertical), then you can visualize the contact between those surfaces is not constant. Instead, the location of the actual contact between these surfaces is constantly changing and as such, there is localized contact followed by localized separation. These repeated compression and separation events are, I theorize, what causes or at least contributes to the metal erosion. |
That was our theory in the past but it has fallen out of favor seeing no evidence of gear cocking. I don't think gear cocking can result in perpendicular erosion.
| Howesight wrote: |
| 6. This is why, with the greatest respect, I don't think that Sodo's suggestion of disengaging 4th gear from time to time will eliminate or substantially reduce this kind of wear. My thinking is that the compression and separation which leads to this erosion is already happening at a tremendous frequency. But the main contribution, I think, is excess torque which increases the amount of axial thrust loading and the amount of gear cocking. |
Some gearboxes have zero thrust surface wear.
Why?
Is it driving style (decels, or no extended highway driving, seldom 4th gear)?
Proper gearbox maintenance?
No electrical assault?
Oil brand?
| Howesight wrote: |
| we will be using Paul Guard's excellent products, including, particulary, his straight-cut 4th gear, drilled mainshaft, new R&P, etc etc. |
I wouldn't use both straight cut and an oiled mainshaft. That's solving the lubrication problem twice.
If my theory is correct, with the GT Gears drilled mainshaft supplying oil to to the 4th gear needle bearing and thrust surface...... you can use helical cut gears.
Quiet is not just more pleasant for the people, it's more pleasant for the bearings too.
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb |
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Pchill2
Samba Member
Joined: November 19, 2021
Posts: 410
Location: Georgia
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| Posted: Thu Sep 07, 2023 10:19 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Sodo what is the diameter of the damage shown on the thrust surface of the 4th gear? Also what is the diameter of the thrust surface of the bearing?
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87 Syncro Westy
Honda K24 swap |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Thu Sep 07, 2023 10:52 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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These features are difficult to measure due to the bevel.
I will try to measure a matched pair.
The ghost appears a bit larger than the diametral slop.
I've been pounding on another (samba)book all morning, trying to correct all the errors etc but I've run out of time and must publish.
I'd be very interested if anyone can labor thru the following post and check my work.
I've been finding and correcting errors so i won't be put off if anyone else finds errors. Honored, actually.
Many of us want to get to the answer to these 094 wear and failure problems, perhaps find some solutions.
Focused collaboration is the best method that we have currently.
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb
Last edited by Sodo on Thu Sep 07, 2023 11:18 am; edited 1 time in total |
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tjet
Samba Member
Joined: June 10, 2014
Posts: 3725
Location: Az
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| Posted: Thu Sep 07, 2023 11:09 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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I'm thinking my transmission failure was caused by a mis-adjusted shifter linkage rod. I thought I had it spot on, but it's possible that 4th gear was only partially engaged.
It did pop out of 4th gear under load 3x before I had no 4th gear. |
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Sodo
Samba Member
Joined: July 06, 2007
Posts: 10617
Location: Western WA
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| Posted: Thu Sep 07, 2023 11:17 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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tjet there are several reasons 4th gear pops out, shimming error perhaps? Worn keystones? A real gearbox builder would know but I don't. It's probably not related to solving the "Big engine + NOT downshifting" but could be related to the axial erosion/damage that allows 4th to travel >>away from the hub (then disengaging ! ).
Howesight, thank you for 'engaging'.
Most people don't wanna engage that Sodo feller or he'll write a danged TL;DR book
Anyway, I hope some of you will read it carefully, as I put a lot of time into it.
I appreciate whatever brain power you are willing to offer.
====== EDIT: Sept 8 ERRORS were made and redacted ! ======
======I left the 'thought process' but added some strike-thrus in hopes that someone can help get it right. ==========
| Sodo wrote: |
| Howesight wrote: |
| 3. Due to the bearing clearance between the mainshaft, rollers and inside diameter (ID) of the 4th gear idler, the idler gear itself can move in an oscillating manner best envisioned by thinking about a hulu hoop in action on somebody's waist; |
I've been calling that motion 'precession'. It's amplitude is equal to the bearing clearance. |
Perhaps the proper name is "Mechanical Precession".
Incidentally, this realization was seeded by Howesight's prescient post back in 2016 which alludes to corrosion fretting but does not use that term.
Samba history here: Syncro Transaxle failed 4th and mainshaft bearings (Pic Heavy.
Paul Guard, Tom Lengyel and I have all repeatedly expressed thanks to Howesight for digging this up, but he won't leave Canada so we can't thank him in person.
Then it took awhile to put it all together because we're not drivetrain engineering professionals.
Strangely, Howesight appears to have departed from his 2016 conclusions - that we have embraced. We think he is mistaken.
From: Wikipedia
AND its not oriented exactly per discussion, but helps visualize how the diametral difference in the gear and shaft may possibly rotate the 4th gear needle bearing rollers/cage.
Someone good with 360Fusion could make us a 'real' animation for our discussion and I bet readers would appreciate that. It could be an 'incremented' rotation which would be difficult to animate.
The green arrow is the 4th gear and mainshaft locked together (see below example 65mph and 3600rpm).
The blue circle (and square) is the needle bearing/cage 'precessing' inside the locked 4th gear (432 RPM below).
OK so this is how it's supposed to work, how the needle bearing does not ruin the shaft and bore immediately. But what stops the needle/cage from rotating. and what starts the longitudinal erosion that the needle settles into? And how could they possibly be "so consistent"?
QUESTIONS>QUESTIONS>QUESTIONS
But in any case we are confident there is non-rotational 'micro-motion' at the thrust surface and the red oxide would not occur unless it was "dry" of oil. We are pretty confident that corrosion fretting is the cause of the thrust erosion and the longitudinal shaft/bore erosion that we call perhaps in error, 'False Brinnelling'. There is no evidence of rotational thrust abrasion.
PChill2 alludes to a resonance condition causing the violent forces to generate erosion at the thrust face. This was one of our past theories too. My theory was not 'at idle' but at somewhere high on the engine's torque curve. Perhaps even keyed to gear tooth vibration (noise). But there was no apparent difference between weddle-big-tooth gears (noisy) and VW fine-tooth gears (quiet), so that didn't go anywhere. We (Tom Lengyel and I) talked about setting up microphones to listen for a resonance condition that "could be audible" at a fairly specific RPM. But not seeing any correlation to tooth-size, we lost interest and never did it.
There could be a resonance that contributes additional erosion.
But the precession has re-risen as the culprit in ~2021, its purely the gear teeth trying to separate, the force shoving the idler gear to the far side of the needle bearing slop
Simple (occam's razor!).
And that amounts to a 'radial' oscillating motion at the thrust face.
The amplitude of this motion = the needle's bearing slop.
One cycle for every rotation of the gear/shaft when locked in 4th.
The needle bearing does 'precess' though. So if nothing is stopping their rolling, the needles rotates slowly around the shaft, burnishing the shaft to a mirror finish, usually NOT sitting in one place on the shaft (which is a blessing ).
Itellya, actually calculating the rotation of the needle inside the locked 4th gear is comforting at some level, for break-in especially.
The needles roll faster than you'd think. Here's simple arithmetic with fairly accurate dimensions approximating RPM of this mechanical precession.
Lets say 65mph is 3,600 RPM.
One rotation of the gear bore is 39.80mm x 3.14 = 125mm circumferential
One rotation of the shaft is 35.00mm x 3.14 = 110mm circumferential
the needle circumference is 2.42mm x 3.14 = 7.6mm circumferential
So the gear bore goes 125-110=15mm farther than the shaft.
Consequently the needle rolls on the shaft, 15mm/7.6mm=2+ turns with every gear rotation.
The tiny needles rolling 2x with every gear revolution; thus at 3600RPM, are rolling 7200 RPM as they shove the cage!
With every gear rotation, the gear bore surface moves 125mm and the needle's center moves 15mm.
Thus the cage is rotating at 15/125 of shaft RPM.
>>>>> At '65mph' the cage is rotating 432 RPM.
That's comforting.
For years I thought the bearing was "stationary", needles in ONE place, one line.
I called Tom Lengyel on the phone a few weeks ago, while zooming down the road in my van, hundreds of miles from home to tell him it has occurred to me that the needle bearing was rolling inside the locked gears.
He replied "hmmmmmm".
I threatened to calculate it.
He siad "hmmmmm" again and recommended that I should do that in his Tom Sawyerism way.
OK here it is. 432 RPM.
======= EDIT Sept 8 =====
432 rpm or 1/8.33 of shaft RPM is outlandish.
(If at all) it has to be a tiny fraction of shaft speed, incremented by “the needle bearing's slop”.
I have not yet found a mechanism for “rotation of the cage”.
The mirror-finish burnishing of the input shaft suggests cage rotation.
But regarding Pchill2’s reply, whatever is happening, it has to be confined within the bounds of “1=1”.
The seldom brinnelling suggests a condition where the slop increments oscillate back&forth in one spot, not advancing.
====================
10cent will tell us its obvious and never assumed that we could get this far being so dumb (now that it comes up). So there's that, can't deny it.
Without 'numbers', it has been a puzzlement how a new mainshaft could get burnished at all. And yet it does. Can't deny it.
How they STOP, though, and then brinnell/erode the shaft and bore is REAL too, and still a puzzlement ----> where we need some bigger brains to come in and save us.
When the shaft and bore are newest, tightest, 'softest", the precession is only a fraction slower. As the shaft and bore wear (more slop, more hard) the needles roll only a tiny bit faster. But if the (perhaps the wrong term!) False Brinnelling lines start to form when dry, the needle could stop rolling, especially if it cannot climb out - and that's bad.
Or perhaps steel trash (from bedding-in a rebuilt) chocks multiple needle simultaneously, especially under "dry" 4th conditions from "sustained helical gear thrust theory". ..... starting the "False Brinnell" erosion?
This is another FLAG to change break-in oil early and often, and certainly DO NOT depart on a long trip on a NEW trans UNTIL after the magnet 'comes clean'. Like 100 miles, 200 miles 500 miles etc. And why a gearbox must be assembled in a "clean room", with ultrasonic parts cleaners, and anal clean-room procedures, etc
---->Not your dirty old garage where your angle-grinder reigns supreme etc.
WHEW! Hope this is understandable.
If I have made any errors please try to explain. (yes there's an error !)
_________________
'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, SyncroShop pressure-oiled pinion-bearing & GT mainshaft, filtered, cooled gearbox.
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered original gearbox
....KTMs, GasGas, SPOT mtb
Last edited by Sodo on Wed Sep 13, 2023 8:54 am; edited 6 times in total |
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tjet
Samba Member
Joined: June 10, 2014
Posts: 3725
Location: Az
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| Posted: Thu Sep 07, 2023 11:37 am Post subject: Re: Big engine + NOT downshifting.... damages 4th gear |
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Yeah you are probably right. But, there is a lot of linkage between the trans and the shifter....And the gears are pretty narrow.
I think if I had some early warning that I was going to get a 4th gear pop-out, I would have stopped to investigate it.
Maybe a proximity sensor on the trans at the shifter lever to activate a warning light?
I know this is not inline with the subject of this thread, but it's too important not to overlook. All the 4th gear cooling and oiling wont help if it's only partially engaged.
I should mention when it did pop out, it did so with enough force to damage the trans. |
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